High aspect ratio layered double hydroxide materials and methods for preparation thereof

ABSTRACT

Embodiments are directed to adamantane-intercalated layered double-hydroxide (LDH) particles and the methods of producing adamantane-intercalated LDH particles. The adamantane-intercalated LDH particles have a general formula defined by [M1-xAlx(OH)2](A)x.mH2O, where x is from 0.14 to 0.33, m is from 0.33 to 0.50, M is chosen from Mg, Ca, Co, Ni, Cu, or Zn, and A is adamantane carboxylate. The adamantane-intercalated LDH particles further have an aspect ratio greater than 100. The aspect ratio is defined by the width of an adamantane-intercalated LDH particle divided by the thickness of the adamantane-intercalated LDH particle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/449,207 filed Mar. 3, 2017, which claims the benefit of U.S.Provisional Application Ser. No. 62/309,645 filed Mar. 17, 2016.

BACKGROUND Field

Embodiments of the present disclosure generally relate to layered doublehydroxide materials, and specifically relate to high aspect ratiolayered double hydroxide materials and methods for preparation.

Technical Background

The synthesis of supported metal or metal-oxide catalysts is of greatindustrial importance in heterogeneous catalysis. High activity, highselectivity, and long catalyst life are desirable characteristics of anyindustrial catalyst. Catalyst materials may be produced from layereddouble hydroxides (LDHs). LDHs, which are also known as anionic clays,are inverse charge analogues of the widely used aluminosilicate cationicclays in their structure and properties. Though a number of methodsexist to prepare metal oxide particles in general, oxides obtained bydecomposing LDHs have several advantages compared to oxide particlesobtained by synthetic methods such as wet impregnation/solid statepreparation. Specifically, LDHs may provide a simple, cost effective andenvironmentally suitable way to achieve a homogeneous distribution ofmetal ions at the atomic level. In order to make use of LDH layers forvarious applications, one needs to be able to exfoliate or delaminatethese layers. Since carbonate LDHs are thermodynamically more stable, itis difficult to exchange it for other ions or to exfoliate LDH layerswhere carbonate is the charge balancing ion. As a result, thesematerials have limited usage. There have been attempts to prepare highaspect ratio non-carbonated LDHs starting from carbonate LDH; however,this approach has multiple steps and is cumbersome.

SUMMARY

In accordance with the background previously presented, ongoing needsexist for LDH materials having high aspect ratios.

Embodiments of the present disclosure are directed to LDHs produced withthe high symmetry adamantane ion, which mediates the growth of highaspect ratio platelets. Moreover, due to its organophilic nature,adamantane can be exfoliated in organic solvents. Thus, these LDH layerscan be used in various applications as previously mentioned. Theembodiments of the present disclosure are directed to high aspect rationon-carbonated LDHs, which use just one equivalent of anion salt.Moreover, these LDHs provide process improvements in that they enable“one pot” synthesis, and less washing (including no washing) at the endof the reaction due to the use of metal hydroxides as starting materialsand just one equivalent of anion. The materials formed also havedesirable properties once calcined.

According to one embodiment, a method for preparingadamantane-intercalated layered double-hydroxide (LDH) particles isprovided. The method comprises adding to an aqueous solution a firstprecursor and a second precursor to form an initial mixture, where thefirst precursor is Al(OH)₃ or Al₂O₃, and the second precursor is ahydroxide M(OH)₂ or an oxide MO, where M is a metal of oxidation state+2. The initial mixture has a M/Al molar ratio of from 1 to 5 or a solidloading of less than 10 weight % solids, based on a total weight of theinitial mixture. The method further comprises adding to the initialmixture an amount of adamantane to form a reaction mixture having anAl/adamantane molar ratio of from 0.5 to 2, and heating the reactionmixture to produce the adamantane-intercalated LDH particles, where theadamantane-intercalated LDH particles have aspect ratios greater than100. The aspect ratio is defined by the width of adamantane-intercalatedLDH particles divided by the thickness of the adamantane-intercalatedLDH particles.

According to another embodiment, an adamantane-intercalated layereddouble-hydroxide (LDH) material in the form of adamantane-intercalatedLDH particles is provided. The adamantane-intercalated LDH particlescomprise a general formula defined by [M_(1-x)Al_(x)(OH)₂](A)_(x).mH₂O,where x is from 0.14 to 0.33, m is from 0.33 to 0.50, M is chosen fromMg, Ca, Co, Ni, Cu, or Zn, and A is adamantane carboxylate. Theadamantane-intercalated LDH particles further comprise an aspect ratiogreater than 100. The aspect ratio is defined by the width of anadamantane-intercalated LDH particle divided by the thickness of theadamantane-intercalated LDH particle.

Additional features and advantages of the described embodiments will beset forth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from that description orrecognized by practicing the described embodiments, including thedetailed description which follows, the claims, as well as the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a Scanning Electron Microscopy (SEM) image of an Mg/Al—CO₃LDH produced via anion exchange;

FIG. 1B is an SEM image of an Mg/Al—CO₃ LDH produced viaco-precipitation;

FIGS. 2A and 2B are SEM images of different magnifications of anMg/Al-adamantoate LDH produced in accordance with one or moreembodiments of the present disclosure;

FIG. 3 is a Powder X-Ray Diffraction (PXRD) graph of a Mg/Al-adamantoateLDH in accordance with one or more embodiments of the presentdisclosure;

FIG. 4 is an Infrared (IR) Spectroscopy graph of a Mg/Al-adamantoate LDHin accordance with one or more embodiments of the present disclosure;

FIG. 5 is a graph of the ¹H solid-state Nuclear Magnetic Resonance (NMR)spectra of a Mg/Al-adamantoate LDH in accordance with one or moreembodiments of the present disclosure; and

FIG. 6 is a graph of the ¹³C solid-state NMR spectra of aMg/Al-adamantoate LDH in accordance with one or more embodiments of thepresent disclosure.

DETAILED DESCRIPTION

The dispersion of active reduced metal or metal-oxide particles on astable support is a complex and laborious process. To achieve this, oneneeds to consider various parameters such as the synthesis conditions,nature of the support and appropriate ways of dispersing/distributingactive catalyst on the support. Among the metal/metal oxide supportedcatalysts, the Cu/ZnO/Al₂O₃ system and metal/metal oxide (Pt, Pd, Rh andAu) systems supported on various supports (alumina, silica, and carbon)have great industrial importance. These catalytic systems are known tohave potential for catalyzing industrially important reactions such assynthesis of methanol, water gas shift reaction, desulfurization ofpetrochemical streams, photochemical/electrochemical splitting of water,and photochemical/electrochemical reduction of carbon dioxide intouseful chemicals, for example.

Reference will now be made in detail to embodiments ofadamantane-intercalated layered double-hydroxide (LDH) particles withhigh aspect ratios and the methods of producing them. Specifically, theadamantane-intercalated LDH particles have aspect ratios greater than100. As defined, the aspect ratio is defined by the width of the LDHparticle divided by the thickness of the LDH particle. As defined, anaspect ratio below 10 is considered low, an aspect ratio less than 100is considered medium, and an aspect ratio of 100 or greater isconsidered a high aspect ratio. The LDH particles may be calculated fromthe SEM images. For example, referring to the embodiment of FIG. 2B, itis clear that the layered particles have large surface area, but lackthickness, thereby resulting in a high aspect ratio. Additionally,Atomic Force Microscopy (AFM) may be utilized to measure the width andthickness of the LDH particles and determine an aspect ratio.

Methods for preparing adamantane-intercalated LDH particles may includethe step of adding to an aqueous solution a first precursor and a secondprecursor to form an initial mixture. In one embodiment, the aqueoussolution may consist essentially of water. The first precursor maycomprise Al(OH)₃ or Al₂O₃. The second precursor may include a metalcontaining compound, for example, a hydroxide M(OH)₂ or an oxide MO,where M is a metal of oxidation state of +2. While various other metalsare also contemplated, the M may be chosen from Mg, Ca, Co, Ni, Cu, Zn,or combinations thereof. In one or more embodiments, the secondprecursor may include Mg(OH)₂, Ca(OH)₂, Co(OH)₂, Ni(OH)₂, Cu(OH)₂,Zn(OH)₂, or combinations thereof. In further embodiments, the secondprecursor is Mg(OH)₂ or MgO. In one example, the second precursor isMg(OH)₂ and the first precursor is Al(OH)₃.

Moreover, in further embodiments, the initial mixture may have a M/Almolar ratio of 1 to 5, or 1 to 3. Furthermore, the initial mixture mayhave a solid loading of less than 10 weight % solids, based on a totalweight of the initial mixture, or a solids loading or less than 5 weight% solids.

Subsequently, the method includes adding to the initial mixture anamount of adamantane to form a reaction mixture having an Al/adamantanemolar ratio of from 0.5 to 2. In one or more additional embodiments, theAl/adamantane molar ratio may be from 0.8 to 1.2, or may be 1 to 1.Various adamantane sources are contemplated. In one embodiment, theadamantane may be added in the form of a carboxylic acid. Optionally,the reaction may be stirred.

Generally, LDHs for conversion to mixed metal oxide catalysts areprepared with inorganic guest anions, which may be easily removed underthermal treatment. When using an organic anion, such as carboxylic acidfunctionalized adamantane, improved properties for LDHs may be achieved.Adamantane has a structure characterized by high symmetry (T_(d)), isfree from intra-molecular strain and, as a result, is extremelythermodynamically stable. At the same time, adamantane can be chemicallyfunctionalized. Adamantane has a melting point of 270° C. and it slowlysublimes even at room temperature. Adamantane is poorly soluble inwater, but readily soluble in hydrocarbons.

Without being bound by theory, the use of thermally stable adamantane isas a structure directing agent, which allows for preferential growth ofthe LDH in the a and b crystallographic directions over the ccrystallographic axes. This results in the high aspect ratio particlesobserved. Moreover, the use of hydrothermal synthesis and metalhydroxide precursors carefully controls the growth conditions in termsof pH and kinetics.

Next, the method includes heating the reaction mixture to produce theadamantane-intercalated LDH particles, where the adamantane-intercalatedLDH particles have aspect ratios greater than 100. As defined, theadamantane-intercalated LDH particles means the adamantane is insertedinto the LDH particle matrix. In further embodiments, the aspect ratioof the adamantane-intercalated LDH particles is greater than 125, orgreater than 150, or greater than 200. Moreover, theadamantane-intercalated LDH particles have a particle diameter of 2 to12 μm, or from 5 to 10 μm. The heating step may occur at a reactiontemperature from 110° C. to 180° C. for a reaction time of 12 hours to48 hours, or from 130° C. to 170° C. for a reaction time of 20 hours to30 hours.

The largest group of the LDH family of materials includes positivelycharged metal hydroxide layers having the composition [M^(II)_(1-x)M^(III) _(x)(OH)₂]^(x+) or [M^(I) _(x)M^(III) _(1-x)(OH)₂]^(x+)(M^(I)=Li; M^(II)=Mg, Ca, Co, Ni, Zn; M^(III)=Al, Cr, Fe; 0.14≤x≤0.33).The positive charge on the layers is balanced by anions present in theinterlayer. The anions give rise to the name anionic clays. One group ofanionic clays includes materials having a general formula [M^(II)_(1-x)M^(III) _(x)(OH)₂](A^(n−))_(x/n).mH₂O or [M^(I) _(x)M^(III)_(1-x)(OH)₂]^(x+)(A^(n−))_(x/n).mH₂O (m=0.33-0.50), where A is an anionsuch as nitrate or halogen.

The adamantane-intercalated LDH particle may have a general formula[M_(1-x)Al_(x)(OH)₂](A)_(x).mH₂O, where x is from 0.14 to 0.33, m isfrom 0.33 to 0.50, M is chosen from Mg, Ca, Co, Ni, Cu, or Zn and A isadamantane carboxylate.

LDHs with high aspect ratios play a role in the development of oxygenbarriers in packaging, as fillers in nanocomposite materials, and asflame-retardants, amongst others. For all these applications, highaspect ratio platelets that can be readily dispersed in a polymer matrixare desirable, but not easily attainable. Anion (charge and symmetry ofthe anion) plays a crucial role in nucleation and growth of LDHcrystals. The carbonate ion, which is ubiquitous in nature, has D₃hsymmetry matching well with interlayer symmetry of the LDH and also ithas higher charge density compared to other anions. As a result, LDHsprefer carbonate ions over other ions and this mediates the orderedstacking of layers. The SEMs of FIGS. 1A and 1B shown Mg/Al-carbonateLDHs produced via anion exchange and co-precipitation respectively.

In contrast to the present embodiments, LDHs are conventionally preparedby a co-precipitation technique, in which a homogeneous mixed solutionof metal salts is added to another solution containing sodium hydroxideand an excess of the guest anion to be incorporated. LDHs obtained fromthis method always show crystallites with submicron size due to rapidmultiple nucleation and crystallization events. Co-precipitated crystalsmay have aspect ratios of approximately 1-10 or less. This indicatesthat the crystals have narrow breadth and grow preferentially along thec axis. This is a reflection of the high supersaturation of both anionand cation and the rapid nucleation of many crystals in the mixed zonein the reactor.

For illustration, SEM images of Mg/Al—CO₃ LDH samples prepared byco-precipitation and anion exchange method are provided in FIGS. 1A and1B. As shown, these particles are irregular thick agglomerates. As aresult of this thickness, the aspect ratio is low for the Mg/Al—CO₃ LDHsamples. In contrast, the SEM micrographs of Mg/Al-adamantoate LDHproduced as shown in FIGS. 2A and 2B depict sheet like layers havingmuch less thickness than the Mg/Al—CO₃. In light of these lowthicknesses, these Mg/Al-adamantoate LDH particles have a high aspectratio.

EXAMPLES

The described embodiments will be further clarified by the followingexample.

Example 1 Preparation of Layered Double Hydroxide

In one typical preparation, a 5 weight % solution of Mg(OH)₂ wasprepared by dissolving 5 grams (g) of Mg(OH)₂ in 95 g of de-ionizedwater. To this 3.36 g of Al(OH)₃ was added to give a Mg/Al molar ratioof 2. Then, 9.31 g of adamantane carboxylic acid was added to the samesolution (Al/adamantane molar ratio=1) and the resultant reactionmixture was stirred vigorously for 1 hour at room temperature. Afterthis, the solution was transferred to a Teflon lined autoclave andheated at 150° C. for 24 hours (h). The pH of the initial reactionmixture and final filtrate was measure and was found to be 9.5 and 8.6respectively. In another set of experiments, the above procedure wasrepeated by taking Mg/Al molar ratio of 5. After the reaction was over,the products were washed thoroughly with water and dried at 65° C.

For comparison, an Mg/Al—NO₃ LDH (Mg/Al molar ratio=2) was synthesizedby a more conventional ammonia precipitation method starting from metalnitrates.

The PXRD pattern of the as-synthesized LDH is given in FIG. 3, and showsthat the basal reflection (001) at 20.84 Å corresponds to a bilayerarrangement of adamantane ions in the interlayer. The submultiples of(001) are seen at higher 2θ values. Intercalation of adamantoic acid wasfurther characterized with IR spectra as shown in FIG. 4. The vibrationsat 1517 cm⁻¹ and 1395 cm⁻¹ correspond to anti-symmetric and symmetricstretching vibrations of the COO⁻ group. The vibrations at 2901 cm⁻¹ and2847 cm⁻¹ are for the C—H vibrations. The 4302 cm⁻¹ vibration is due tohydrogen bonding of layer metal hydroxide groups with intercalated watermolecules in the interlayer.

The ¹H and ¹³C solid-state NMR spectra of Mg/Al-adamantoate LDH wererecorded and are given in FIGS. 5 and 6, respectively. The 4 sharp peaksin the ¹H spectra of FIG. 5 at lower ppm values are due to the hydrogenspresent in the adamantane ring. The peaks at 3.8 ppm and 4.8 ppm are dueto the hydrogens of the intercalated water and metal hydroxiderespectively. Referring to FIG. 6, the ¹³C NMR spectra ofMg/Al-adamantoate shows 4 peaks at 29.5 ppm, 37.3 ppm, 40.6 ppm and 42.8ppm, which are due to 4 different carbons present in the adamantanemolecule. The peak at 186.98 ppm is due to the carbon of the carboxylategroup. Scanning Electron Microscope (SEM) images of as-synthesized LDHshow platelet morphology typical of layered materials (FIGS. 2A and 2B).

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments describedherein, provided such modification and variations come within the scopeof the appended claims and their equivalents.

What is claimed is:
 1. An adamantane-intercalated layereddouble-hydroxide (LDH) material in a form of adamantane-intercalated LDHparticles, where the adamantane-intercalated LDH particles comprise: ageneral formula defined by [M_(1-x)Al_(x)(OH)₂](A)_(x).mH₂O, where x isfrom 0.14 to 0.33, m is from 0.33 to 0.50, M is chosen from Mg, Ca, Co,Ni, Cu, or Zn, and A is adamantane carboxylate; and an aspect ratiogreater than 100, the aspect ratio defined by a width of anadamantane-intercalated LDH particle divided by a thickness of theadamantane-intercalated LDH particle.
 2. The adamantane-intercalated LDHmaterial of claim 1 where M is Mg.
 3. The adamantane-intercalated LDHmaterial of claim 1 where the aspect ratio is greater than
 125. 4. Theadamantane-intercalated LDH material of claim 1 where the aspect ratiois greater than
 150. 5. The adamantane-intercalated LDH material ofclaim 1 where the aspect ratio is greater than
 200. 6. Theadamantane-intercalated LDH material of claim 1 where theadamantane-intercalated LDH particles have a particle diameter of 5 to10 μm.
 7. The adamantane-intercalated LDH material of claim 1 where theadamantane-intercalated LDH particles have characteristic peaks in an IRspectra at 1517 cm⁻¹, 1395 cm⁻¹, 2901 cm⁻¹, 2847 cm⁻¹, and 4302 cm⁻¹.